Mamiya made a duel cable release. One press locks the mirror, a second press triggers the shutter. They show up on Ebay from time to time and are a little pricey.

I am certain that those were actually developed for the K/L Lenses on the RB. All K/L Lenses have two cable release sockets, one for opening, one for closing the shutter. You would screw both of the cables from the dual-cable release into the lens, trip the mirror by hand, wait, then open and close with the release. A lot better than having to close the shutter via the dial or the cocking lever which you had to do with the older non-c, NB, and C series lenses.